The present invention relates to a two-way radio communication system of the type commonly known as "mobile radio". More particularly, the invention relates to a method and apparatus for establishing communication in such a system on a common radio communication "channel" or frequency selected from a plurality of such channels.
As used herein, the term "radio channel" is intended to include a first frequency for transmitting and a second, different frequency for receiving by a plurality of user-operable radio transmitter/receiver stations. Signals transmitted by such user operable stations at the first frequency are received by a nearby radio repeater station and then retransmitted at the second frequency for reception by all the user-operable stations in the vicinity that are tuned to the channel.
As used herein, the term "mobile radio" is intended to describe the type of two-way radio system commonly used to communicate between and among a plurality of mobile or portable transmitter/receiver stations and one or more fixed base transmitter/receiver stations, with the aid of a repeater station. The mobile or portable stations are arranged on mobile vehicles of all types, such as motor vehicles and boats, or hand held. A typical application of such a system is to dispatch and control the operation of taxis, tow vehicles, repair vehicles, delivery vehicles and the like. Normally, fleets of such mobile vehicles are owned and operated by separate business entities, and it is advantageous if all, or a subset of, vehicle operators in a common fleet or business are able to communicate with each other and with a dispatcher at a fixed base station. Such a set of users who can communicate with each other by means of the mobile radio system will be denoted hereinafter as a "group" of users. It is further advantageous if the vehicle operators and fleet dispatcher of one fleet or business (one group of users) are prevented from communicating with, either by transmitting to or receiving from, vehicle operators and dispatchers of other fleets or businesses (other groups of users). Thus, the mobile radio system advantageously operates as a "party line" within each group or subgroup of users, and "private line" as between groups or subgroups of users.
In two-way mobile radio communication systems of this type a plurality of radio channels are allocated for communication between al user-operable radio stations. Generally there may be, and usually are, many more radio stations than there are available radio channels in a given locality. Consequently, some arrangement is needed for enabling the communication among a large number of users, in the party line/private line basis described above, on a relatively small number of channels.
The U.S. Pat.No. 4,409,687 to Berti et al. describes an arrangement for establishing communication in a mobile radio environment which utilizes a "busy tone" in the middle of the voice band of each channel. Each user-operable station in the system scans the available channels, in either a "transmit mode" or "receive mode", and takes action in dependence upon the presence (or absence) of a busy tone. Each radio station utilizes a "notch filter" to remove the busy tone from the voice signal before the voice signal is processed. This arrangement is incompatible with other existing equipment in the field since it requires that all radio stations be able transmit, detect and remove a busy tone from the voice band in each channel.
Mobile radio systems, as presently in common use, distinguish between groups of users by so-called "sub-audible group tones"; that is tones in the voice band below 300 Hz. Such tones are variously called "CTCSS" and "DCSS" tones for "continuous tone coded squelch system" and "digital coded squelch system", respectively. In certain mobile radio channels in the United States--that is, in the radio channels allocated in the 450 to 470 MHz. range--there are a total of 38 CTCSS tones and 215 DCSS tones closely spaced in the frequency range between 65 and 250 Hz. The CTCSS tones are simply continuous tones whereas the DCSS tones repeatedly provide seven bit words that convey the identity of a group. Each mobile radio station includes equipment for detecting the presence of absence of selected CTCSS tones and/or for decoding the transmitted DCSS words. Although the use of CTCSS and DCSS tones is strictly voluntary on most frequencies, most mobile radios manufactured today are equipped to generate and detect one type of tone or the other, or both.
It is thus common practice to create "private lines" between groups of users, and "party lines" among groups of users, by selecting a CTCSS or DCSS tone on each separate channel. The CTCSS or DCSS tone need not be the same for each channel provided that the radio stations of all users in a given group have been assigned the same tone for each channel.
Channel selection in mobile radio systems of this type has traditionally been accomplished by manually adjusting the channel selector or by scanning. Automatic channel selection, for example as described in the aforementioned U.S. Pat. No. 4,409,687, has not been thought possible because of the complexity of the system environment that contains mobile radio stations which are not equipped to handle the "busy tone" format.
As used herein, the term "trunking", as used in the mobile radio environment, is intended to describe the automatic selection of an available channel, when in the transmit mode, and the automatic acquisition of a calling channel, when in the receive mode.